Bottom Line:
Here we demonstrate the retrieval of multiple bond lengths from a polyatomic molecule by simultaneously measuring the C-C and C-H bond lengths in aligned acetylene.Our approach takes the method beyond the hitherto achieved imaging of simple diatomic molecules and is based on the combination of a 160 kHz mid-infrared few-cycle laser source with full three-dimensional electron-ion coincidence detection.Our technique provides an accessible and robust route towards imaging ultrafast processes in complex gas-phase molecules with atto- to femto-second temporal resolution.

f2: Method to extract structural information from the momentum distributions.(a) Logarithmically scaled momentum distribution of electrons corresponding to all ionic fragments. The circles represent the scattering of electrons with the same energy at different angles. (b) The detected ion TOF showing the numerous fragments created during the strong-field interaction. The inset shows the peak corresponding to the C2H2+ ion near 4.2 μs and the shaded region represents the window of ions that the C2H2+ electrons are taken from. (c) The electron kinetic energy distribution for the C2H2+ ion (black) and for all possible fragmentation processes (blue). (d) An extracted MCF for the acetylene cation (black circles) as well as for electrons from all fragments (blue squares). The solid black curve shows the best fit, which matches very well with the cation channel. The MCFs for ±10% changes in the C2H2 molecular lengths (dashed curves) highlight the sensitivity of the LIED technique. The s.d. error bars are derived from Poissonian statistics.

Mentions:
The procedure for extracting structural information from aligned (see Supplementary Note 1 and Supplementary Figs 1 and 2) C2H2 using LIED is outlined in Fig. 2. Figure 2a shows the momentum distribution of all electrons detected in coincidence with all positive fragments after the ionization of C2H2 with our mid-infrared source. Following the quantitative rescattering theory (see Supplementary Note 2), the molecular differential cross-section (DCS) is extracted by sweeping the scattering angle (θr) around the circumference of a circle with radius equal to the momentum of the rescattered electron (kr). The influence of the ionizing laser field must be considered; consequently, the origin of the circle is given by the vector potential (Ar) at the time of rescattering (see the Supplementary Note 3 and Supplementary Fig. 3). Each circle represents rescattering by different electron energies. The extracted experimental molecular DCS (σM) is combined with the theoretical atomic DCS (σA), which is calculated using the independent atom model (see the Supplementary Note 4), for the same electron energy and emission angle to calculate the molecular contrast factor (MCF) MF=(σM–σA)/σA. The MCFs are typically presented as a function of the momentum transfer q=2kr sin(θr/2) experienced by the rescattered electrons. A χ2-based fitting routine is used to compare the experimentally obtained MCF to theoretical predictions (see the Supplementary Note 5 and Supplementary Fig. 4).

f2: Method to extract structural information from the momentum distributions.(a) Logarithmically scaled momentum distribution of electrons corresponding to all ionic fragments. The circles represent the scattering of electrons with the same energy at different angles. (b) The detected ion TOF showing the numerous fragments created during the strong-field interaction. The inset shows the peak corresponding to the C2H2+ ion near 4.2 μs and the shaded region represents the window of ions that the C2H2+ electrons are taken from. (c) The electron kinetic energy distribution for the C2H2+ ion (black) and for all possible fragmentation processes (blue). (d) An extracted MCF for the acetylene cation (black circles) as well as for electrons from all fragments (blue squares). The solid black curve shows the best fit, which matches very well with the cation channel. The MCFs for ±10% changes in the C2H2 molecular lengths (dashed curves) highlight the sensitivity of the LIED technique. The s.d. error bars are derived from Poissonian statistics.

Mentions:
The procedure for extracting structural information from aligned (see Supplementary Note 1 and Supplementary Figs 1 and 2) C2H2 using LIED is outlined in Fig. 2. Figure 2a shows the momentum distribution of all electrons detected in coincidence with all positive fragments after the ionization of C2H2 with our mid-infrared source. Following the quantitative rescattering theory (see Supplementary Note 2), the molecular differential cross-section (DCS) is extracted by sweeping the scattering angle (θr) around the circumference of a circle with radius equal to the momentum of the rescattered electron (kr). The influence of the ionizing laser field must be considered; consequently, the origin of the circle is given by the vector potential (Ar) at the time of rescattering (see the Supplementary Note 3 and Supplementary Fig. 3). Each circle represents rescattering by different electron energies. The extracted experimental molecular DCS (σM) is combined with the theoretical atomic DCS (σA), which is calculated using the independent atom model (see the Supplementary Note 4), for the same electron energy and emission angle to calculate the molecular contrast factor (MCF) MF=(σM–σA)/σA. The MCFs are typically presented as a function of the momentum transfer q=2kr sin(θr/2) experienced by the rescattered electrons. A χ2-based fitting routine is used to compare the experimentally obtained MCF to theoretical predictions (see the Supplementary Note 5 and Supplementary Fig. 4).

Bottom Line:
Here we demonstrate the retrieval of multiple bond lengths from a polyatomic molecule by simultaneously measuring the C-C and C-H bond lengths in aligned acetylene.Our approach takes the method beyond the hitherto achieved imaging of simple diatomic molecules and is based on the combination of a 160 kHz mid-infrared few-cycle laser source with full three-dimensional electron-ion coincidence detection.Our technique provides an accessible and robust route towards imaging ultrafast processes in complex gas-phase molecules with atto- to femto-second temporal resolution.